Human beings from two and a half millennia ago stand before us, sporting road warrior haircuts, designer beards and skin with tattoos of outlandish predators.

These people are from the pre-historic forest-steppes of southern Siberia, and up close we can almost feel their emotions, their lives.

They are the Scythians, a great people of nomadic, mounted conquerors, a possible model for Game of Thrones’ Dothraki warriors, the predecessor to Genghis Kahn.

This British Museum exhibit also happens to be sponsored by British Petroleum.I strolled by the museum Saturday with 50 black-clad local activists, members of ‘BP or not BP’ and with my friends from New York, the Stop Shopping Choir. We entered the museum in twos and threes just before noon. We walked the length of the vast building, all the way to the gates of the Scythian show – where the mummified warriors, the new King Tuts of historical spectacle, await the crowds.

On a signal, and by now surrounded by museum cops in their fluorescent vests, I began my preaching. I had to ask… how can a company sponsor something that it is destroying? Climate change is threatening to shut the archaeological excavations on which the exhibition is based.

The melting permafrost is washing away the remains of the Scythian culture. BP – such a huge extractor of the fossil fuels burning up our planet – seems to want to present the last artefacts that the archaeologists can get out of the lost empire, while destroying the future of the dig.

The UK activists and the choir got down – this had been rehearsed this before our visit – on the white marble floor of the museum and pull themselves into (recyclable) black plastic bags, with rips in them so that their fingers and elbows and faces are poking through.

The effect is that there appear to be lots of people strewn across the floor of the museum who are drowned in oil. Savitri D, co-founder of the Stop Shopping Choir, designed this unmuseum-like tableau, and on the floor herself, directs this struggling mass of humanity. Nehemiah Luckett, the music director of the choir, is singing Yeats’ apocalyptic words of ‘The Second Coming’: ‘Things fall apart... the best lack all conviction.’

The volunteers push and pull their black plastic on the floor, and suddenly the illusion of a spreading oil slick on the museum’s floor is conjured.

We sing and move across the vast space of the British Museum, walking in slow, nearly still motion behind a receding shoreline of glistening black. We walk, harmonizing.

Then we start the old Digger song, ‘Stand Up Now’ but with our new lyrics, ‘Keep the oil in the ground, in the ground, in the ground…’

People follow us in our ritual walk from the Scythian show to the front door of the old museum. We are in a trance, the oil pulling away, leaving us to dream of being free of oil. It pushes back, before receding again.

Other museum-goers walk with us. A lady in a wheelchair joins our procession, children, straight-up tourists. People watch from the balconies above; families rise from the tables over by the gift store, as the oil is pushed and pulled from the building.

Writing this now, I feel something special happened in the slowness of our walk across that expanse. We walked and sang as the oil receded before us. Where were we? In a colonial symbol, the British Museum? On the wind-swept steppes, in the gulf with the Deepwater Horizon?

We were walking so slowly that our prayer had a strange interior quality, a crowd softly moaning its belief. The oil moved toward the door.

When the corporate wreckers of our planet purchase prestige, how do we recapture our institutions? Our ritual gave people a music that clashed with the spaced-out monotone of the spectacle of bones and axes.

We hope that soon it will be harder to be so unthinking that a company is permitted to end a famous archaeological dig while taking a starring role in its last exhibition. Do we let the killers pose with the victims before the murder? Climate change is like that. It is a new colonialism. We stop that. We stop shopping for that picture.

Rev Billy and the Stop Shopping Choir have just finished their nine city ‘Trump Depression Hotline Tour’ at the British Museum. They live in New York City where they often take rituals to Trump Tower. The ‘BP or not BP’ campaign is ongoing and welcomes your support.